This invention relates generally to high data rate optoelectronic networks, and, more particularly, to a multi-fiber, in-line attenuator module and assembly configured for insertion in the fiber optic pathway of an optoelectronic network between a semiconductor laser and a semiconductor detector to attenuate the optical power levels by a predetermined value for all propagated modes, i.e., attenuation is mode independent.
The need for greater information bandwidth has led to the increased use of optoelectronic networks operating at and above 1 gigabit/s, i.e., the Gigabit Ethernet. And, in particular, the 10-Gigabit Ethernet is a developing optoelectronic technology that offers data speeds up to 10 billion bits (gigabits) per second. It is not uncommon in such high-speed optoelectronic networks to have a requirement for a fixed value of attenuation in the fiber optic pathway, most typically to prevent saturation or overloading of a semiconductor detector. A fixed-value attenuation requirement can also be imposed in branched optoelectronic networks to balance optical power levels in the different branches.
Such an attenuation requirement can be fulfilled by an adjustable attenuation mechanism or a fixed value attenuation mechanism. Traditional attenuation mechanisms include a separation or gap (air) between adjacent fibers (gap-loss attenuation), where such separation attention mechanisms may be either fixed value, see, e.g., U.S. Pat. No. 5,701,382, or adjustable, see, e.g., U.S. Pat. No. 5,734,778, lateral offset or core mismatch between adjacent fibers, or the introduction of blocking material between adjacent fibers.
Achieving a fixed-value attenuation requirement in high-speed optoelectronic networks, however, using one of these fixed value attenuation mechanisms becomes more complex due to the use of multimode fibers and laser light sources in such networks. High-speed optoelectronic networks tend to use multimode fibers for short to moderate transmission distances, e.g., 550 to 2,000 meters (single mode fibers being used for longer transmission distances) due to cost considerations and integration with pre-existing multimode fiber infrastructures. Multimode fibers are designed to carry multiple electromagnetic modes concurrently, each electromagnetic mode having a slightly different reflection angle within the fiber core. Multimode fibers having core diameters of 50 and 62.5 microns are the current standards for high-speed optoelectronic networks.
While LEDs have heretofore been used as the light sources for fiber optic telecommunication systems, LEDs cannot achieve the high data rate speeds inherent in the Gigabit Ethernet. Therefore, the Gigabit Ethernet uses semiconductor laser diodes. Of particular interest for Gigabit Ethernet applications is the vertical cavity surface emitting laser (VCSEL), a specialized laser diode constructed to emit energy at 850 nm and 1300 nm. Although VCSELs are cost effective devices for high-speed multimode fiber transmission applications, there are notable differences in launch characteristics, e.g., size, shape, and power distribution, among VCSELs produced by different manufacturers.
In addition, VCSEL devices exhibit orthogonal polarization states at and above their threshold currents as well as unstable polarization switching, which results in an increase in modal noise. If fiber length is short, as it tends to be in multimode applications, modes do not have time to equalize. VCSEL transmission launch conditions vary over time, leading to excitation of random modes. Launching a laser into multimode fiber generates multiple modes that are subject to differential mode delay, i.e., different propagation times, which adversely affect detector performance.
The foregoing described conditions can lead to random mode excitements in multimode fiber networks such that the mode distribution in the network is a random variable. A random-variable mode distribution, in turn, leads random attenuation characteristics in multimode fibers and/or in-line attenuation mechanisms. Random attenuation characteristics can result in variations in the detected signal levels at the detector, which adversely affects the integrity and reliability of the detector output.
A need exists to provide an attenuation device for use in optoelectronic networks that provides mode-independent attenuation. Such an attenuation device should also be configured to provide a predetermined value of attenuation, depending upon the particular application. The attenuation device should also be easily reconfigurable so that different predetermined values of attenuation can be readily provided for different applications.
One object of the present invention is to provide a multi-fiber, in-line attenuator module and assembly that provides mode independent attenuation of all modes propagated in an optical pathway.
Another object of the present invention is to provide a multi-fiber, in-line attenuator module and assembly that provides a predetermined value of attention of all modes propagated in an optical pathway, where such predetermined value is a specification of a particular application.
These and other objects are achieved by a multi-fiber in-line attenuator module according to the present invention that is configured for insertion in the fiber optic pathway of an optoelectronic network to provide a predetermined value of attenuation for all propagating modes in the pathway, the module including first and second multi-channel interface members such as ferrules, V-groove arrays, PLC members or combinations thereof, each having a mating face, an interconnect face, and alignment holes and n-optical channels formed therethrough, a multi-fiber ribbon cable terminating in the interconnect face of each multi-channel interface member with the optical fibers thereof disposed in the n-optical channels, alignment pins disposed in the alignment holes of the first and second multi-channel interface members so that the n-optical channels of the first and second multi-channel interface members are optically aligned, a mating clip for retaining the first and second multi-channel interface members in mated combination, and an NDF (neutral density filter) film adhered to at least one of the mating faces of the first and second the multi-channel interface members, the adhered NDF film having a predetermined composition and thickness to provide the predetermined value of attenuation for the multi-fiber in-line attenuator module.